1. Field of the Invention
The present invention relates to a laser system such as an excimer laser, and more particularly, to a system and method for detecting and controlling a specific wavelength of a laser beam.
2. Description of the Related Art
Generally, a laser such as an excimer laser is applied widely for semiconductor fabrication. An output wavelength of the excimer laser should be accurately controlled to have a constant value as a function of time and operation state. Particularly, when a semiconductor integrated circuit is manufactured, an accurate control of the output wavelength is essential.
An opto-galvanic sensor has been used in a method of controlling an accurate wavelength. The opto-galvanic sensor uses resonance between an incident laser beam and plasma. Resonance is caused by a cathode and gas filled within the sensor. That is, the opto-galvanic sensor uses a property in which an electrical characteristic of the plasma is varied when a wavelength of the laser beam resonates with a specific absorbed wavelength of atom and molecule of the plasma. However, a method of detecting the wavelength of the laser beam by using the opto-galvanic sensor has a disadvantage in that it is difficult to accurately arrange the sensor and the laser beam, and it is difficult to use a diffuser. In order to solve the above drawback, a Hollow Cathode Lamp (HCL) is used to measure an amount of the laser beam absorbed by a volatile material disposed inside a vacuum chamber of the sensor.
FIG. 1 is a schematic view illustrating a conventional system of detecting a wavelength of the laser beam by using the hollow cathode lamp.
Referring to FIG. 1, a laser system 10 includes a laser 13, a wave meter 11 for sampling a laser beam (A) and determining a wavelength of the sampled laser beam (A), and a wavelength controlling unit 15 for selectively controlling the wavelength of the laser beam (A) at a specific wavelength range. The wave meter 11 controls the wavelength thereby controlling unit 15. That is, if the detected laser beam (A) does not have the same wavelength as desired, the wave meter 11 sends a correction signal to the wavelength controlling unit 15 to control the value of the wavelength of the detected laser beam (A).
The laser beam (A) passing through the wave meter 11 is divided into a first beam (A1) and a second beam (A2) by using a first beam splitter. The second beam (A2) passes through a diaphragm 34 and a diffuser 32 for widening the width and the span of the beam to be incident with the wavelength detecting system having the hollow cathode lamp 40.
The wavelength detecting system includes the hollow cathode lamp 40, a current controlling unit 47 and an optical detector 49. The hollow cathode lamp 40 includes a vacuum chamber 42, an anode 43, and a cathode 44 disposed inside the vacuum chamber 42. The vacuum chamber 42 includes an input window 41 for inputting the incident laser beam (A2) and an output window 45 for outputting the laser beam (A2). Here, the cathode 44 is generally formed of a cylindrical volatile metal material, for example, iron (Fe). The vacuum chamber 42 is filled with gas such as neon.
The current controlling unit 47 controls the amount of current flowing through the anode 43 and the cathode 44. The volatile material fills the cathode 44 through which the second beam (A2) passes. Gas volatilized from the volatile material and an inert gas such as neon form the plasma to absorb the second beam (A2) at a wavelength determined by a characteristic of the plasma. For example, if iron is the volatile material it absorbs the laser beam with a maximum wavelength of about 248 to 3271 nm.
Some of the absorbed laser beam (A2) is transmitted through the output window 45, and the optical detector 49 detects the amount of the transmitted beam. The detected amount of the beam depends on the wavelength of the second beam (A2). The optical detector 49 relatively detects the amount of beam with a minimum wavelength of about 248 to 3271 nm. The detected amount of beam is transmitted to the first controller 50, and the first controller 50 again controls the wavelength of the laser beam (A) emitted from the wave meter 11 and from the wavelength controlling unit 15.
Meanwhile, the first beam (A1) passing through the first beam splitter 30 is transmitted to a reticle 68, after passing through a beam conditioner 60, a beam condenser 62, a second beam splitter 64 and a lens 66. The optical convergence unit 70 irradiates the beam passing through the reticle 68 onto a wafer 82 seated on a stage 80. The stage 80 moves in an X-axis and a Y-axis in the same plane by using an X-Y controller 84.
Further, the first beam (A1) passes through the optical convergence unit 70 to become a third beam (A3) with intensity and uniformity varied. The third beam (A3) is substantially irradiated on the wafer 82, and its intensity and uniformity is detected using a beam sensor 90 before it is irradiated on the wafer 82. The detected result is used to allow a second controller 92 to again determine the laser beam (A) emitted from the laser system 10.
However, the beam sensor 90 can check the intensity and the uniformity of the third beam (A3), but cannot detect a specific wavelength of the third beam (A3) irradiated on the wafer 82. That is, it cannot check whether or not the third beam (A3) which directly irradiated the wafer 82 has the specific wavelength. Further, the conventional specific wavelength detecting sensor should include separate devices such as the current controlling unit 47 and the optical detector 49 for driving the detection process. Accordingly, the conventional specific wavelength detecting sensor has a difficulty in simplifying the equipment, and is disadvantageous in an economic aspect.